1. Field of the Invention
Example embodiments generally relate to a lighting device, an optical scanning unit, an image reader, and an image forming apparatus, and more particularly, to a lighting device for irradiating an original document sheet, an optical scanning unit, an image reader, and an image forming apparatus including the lighting device, for example.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, image scanners, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a sheet) according to image data using electrophotography. Thus, for example, an image reader reads an image on an original document sheet to generate image data; a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner particles to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a sheet or is indirectly transferred from the image carrier onto a sheet via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the sheet; finally, a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet, thus forming the image on the sheet.
In such image forming apparatuses, the image reader may include a lighting device in which a light-emitting diode (LED) is used as a light source to provide quick warm-up, energy saving, and a long life of the light source.
However, the LED has a small light-emitting surface, roughly equivalent to a point source, and therefore does not provide uniform illuminance distribution in a sub-scanning direction. To address this problem, related-art lighting devices may include a guide member to guide light emitted by the LED.
Thus, for example, a related-art image reader may include a lighting device including a plurality of point sources, a guide member formed of transparent resin or glass, and a positioning member. The plurality of point sources is aligned so as to irradiate an original document sheet through an exposure glass, with the guide member guiding light emitted by the plurality of point sources to irradiate the original document sheet in a main scanning direction. The positioning member positions the plurality of point sources with respect to the guide member in such a manner that a constant gap is provided between the plurality of point sources and the guide member, and a direction in which the plurality of point sources is aligned corresponds to a longitudinal direction of the guide member.
However, some original documents present special problems. Thus, for example, when an open book or a thick sheet is placed on the exposure glass, a center portion of the open book lifted from the exposure glass, or a step formed between the thick sheet and the exposure glass, may result in a shaded image of the original.
To address this problem, some image readers include first and second lighting devices which include first and second light sources and first and second guide members for guiding light emitted by the first and second light sources to an original document sheet, respectively. The first lighting device is provided upstream from an irradiation region in which light emitted by the first light source irradiates the original document sheet in the sub-scanning direction, and the second lighting device is provided downstream from the irradiation region in the sub-scanning direction.
However, although such an arrangement can solve the problem described above, in such an image reader, the light sources need to be positioned precisely with respect to the guide members to obtain proper illuminance distribution, thus complicating maintenance of the image reader.